This invention relates to an optical head which can be employed in an optical disk device, an optical video-disk device, a compact-disk player and so on.
FIG. 1 shows a plane view showing a construction of a conventional optical head which is employable in an optical disk device. In the drawing, numeral 1 indicates a bobbin serving as a lens holder constructed to mount an objective lens 2 thereon. The bobbin 1 is arranged to the movable along a direction indicated by an arrow "A" as well as a direction indicated by a pair of symbols "B1" and "B2" which is orthogonal to the "A" direction, i.e., perpendicular to a surface of a sheet on which the drawing is shown. A focusing coil 3 is wound around the outer surface of the bobbin 1, and tracking coils 4A through 4D are mounted on the focusing coil 3. Further, numerals 5, 5 indicate a wire suspension mechanism for supporting the bobbin 1. One pair of terminals of the wire suspension mechanism 5 is connected to an outer surface of the bobbin 1, while the other pair of terminals thereof is connected to a fixing member 6 which is fixed within a device in which the optical head to be provided. Therefore, the bobbin 1 is supported by the suspension mechanism 5 and is moved in the "A" direction as well as "B1-B2" direction in accordance with deformation of wires comprising the wire suspension mechanism 5.
Numerals 7A through 7D respectively indicate permanent magnets mounted on fixing members 12A through 12D for generating a magnetic field. Thus, the focusing coil 3 and the tracking coils 4A through 4D are located in the magnetic field generated by the permanent magnets 7A through 7D. Numeral 8 indicates a reflecting plate for reflecting light transmitted from light transmitting elements included in a pair of optical sensors 9A and 9B, described later. The reflecting plate 8 is mounted on the bobbin 1, as shown on FIG. 1, the opposite side surface with respect to the surface on which the suspension mechanism 5 is connected as shown in FIG. 1. The reflecting plate 8 is arranged, for example, such that the surface of a metal, glass and so forth ground in advance, is evaporated by a thin metal membrane. Further, a pair of optical sensors 9A and 9B are provided opposite to the reflecting plate 8. A differential operational amplifier 10 is provided for outputting signal in accordance with the difference between the outputs of the optical sensors 9A and 9B.
As shown in FIG. 2, showing a partial enlarged view of the FIG. 1, the pair of optical sensors 9A and 9B respectively include light transmitting elements 9A1 and 9B1, such as LEDs (Light Emitting Diodes), and light receiving elements 9A2 and 9B2, such as PDs (Photo Diodes). The light transmitted from the light transmitting elements 9A1 and 9B1 respectively illuminate edge portions of the reflecting plate 8 as shown in FIG. 2.
In the above structured conventional optical head, when a signal caused by a moving error of the optical head along a focusing direction, i.e., "B1-B2" direction, is supplied to the focusing coil 3, a moving force is generated along the "B1-B2" direction, in accord with Fleming's left hand rule, since the focusing coil 3 is located in the magnetic field generated by the permanent magnets 7A through 7D. Accordingly, the wires of the suspension mechanism 5 are deformed, and then, the optical head can be moved along the "B1-B2" direction. On the other hand, when a signal caused by the moving error of the optical head along a tracking direction, i.e., "A" direction, is supplied to the tracking coils 4A through 4D, moving force is generated along the "A" direction in accord with Fleming's left hand rule, and then, the optical head can be moved along the "A" direction. When the optical head is moved upwardly in the drawing of FIG. 2, an amount of the light reflected by the reflecting plate 8 transmitted from the light transmitting element 9A1 is increased. Therefore, the amount of light received by the light receiving element 9A2 is increased. In this case, the amount of light received by the light receiving element 9B2 is decreased. On the contrary, when the optical head is moved downwardly in the drawing, the amount of light received by the light receiving element 9B2 is increased, and the amount of light received by the light receiving element 9A2 is decreased. Accordingly, by detecting the difference between the outputs "A" "B" of these light receiving elements, which corresponds to the amount of received light, it becomes possible to determine the position of the optical head along the "A" direction, i.e., the tracking direction.
In the above conventional optical head, since the surface of the reflecting plate 8 is arranged to be evaporated by thin metal membrane on a surface of a metal, glass and so forth having been ground in advance, the cost for obtaining the above structured reflecting plate 8 is increased. Further, since a surface of the reflecting plate 8 is specularly arranged for effectively reflecting light, as illustrated in FIG. 3, an area "1", in which linearity between movement of the optical head along the tracking direction, and the output level of the differential operational amplifier 10, is substantially satisfied, can not be increased. Furthermore, the output level of the differential operational amplifier 10 is undesirably varied, if a crack, chipped portion and so forth exists at the edge portions of the reflecting plate 8.